Chapter 5 Experimental Investigation of the Dynamic Characteristics of a Glass-FRP Suspension Footbridge Xiaojun Wei, Justin Russell, Stana Živanovic´, and J. Toby Mottram Abstract Due to high strength- and stiffness-to-weight ratios, good durability performance in a variety of environments and quick installation, fibre reinforced polymers have increasingly been utilised for construction of highway and pedestrian bridges. Their relatively low mass and stiffness make these bridges potentially susceptible to vibration serviceability problems, which are increasingly governing the design. Currently, a lack of experimental data on the dynamic characteristics of polymeric composite structures is hindering their wider application and the development of design guidance. To fully exploit the benefits of using these structural materials in bridge engineering requires a better understanding of their dynamic behaviour. The aim of this paper is to utilise ambient vibration measurements to experimentally identify the dynamic characteristics (i.e., natural frequency, damping ratio and mode shape) of a glass fibre reinforced polymer deck suspension footbridge in the UK. It is found that the Wilcott footbridge possesses a relatively high density of vibration modes in the low frequency range up to 5 Hz and has damping ratios of most of these modes >1%. Keywords FRP suspension footbridge • Dynamic characteristics • Ambient vibration testing • Peak picking method • Stochastic subspace identification method 5.1 Introduction Since the first Fibre Reinforced Polymer (FRP) road bridge was built in Miyun, China in 1982 [1], thousands of bridges with FRP components have been built around the World [2]. The driver for this practice is this structural materials’ favourable properties, including: high strength- and stiffness-to-weight ratios, good durability and short installation time. FRP construction is typically employed for short-to-medium span bridges. The longest span to date is 63 m, achieved with the Aberfeldy footbridge that was constructed in Scotland in 1992 [3], using the same construction system as for the Wilcott footbridge. The benefits of using FRP as the structural material would be more prominent if longer bridge spans could be executed. Due to the lightweight and relatively low stiffness of glass FRPs, FRP bridges may be very lively and potentially suffer excessive vibration, causing user discomfort and affecting the bond in joints and between surfacing and the FRP superstructure [4]. Vibration serviceability is increasingly found to govern the design of FRP structures. A sound knowledge and understanding of their dynamic characteristics is therefore important for us having robust serviceability design procedures. Owing to a lack of experimental data on the dynamic characteristics we find that existing design guidance used in conventional material designs is usually employed. This approach may produce a conservative solution and compromise the benefits of using FRP in the first place. For example, in the AASHTO Guide Specifications for Design of FRP Pedestrian Bridges (1st Ed.) [5], a damping ratio of 2–5% is recommended. A damping ratio of at least 1% is expected to be present in every FRP structures, whilst such a high value as 5% is rarely achieved with pedestrian bridges made of other structural materials, such as steel, steel-concrete composite, aluminium and, even, concrete. In Prospect for New Guidance in the Design of FRP [6] the damping ratio is specified to be 1.5% for a conservative estimate in design analysis, and higher damping values may be used if these have been substantiated by representative experimental data. Given that the above recommendations for damping ratios are not necessarily based on extensive and comprehensive data from as-built FRP X.Wei ( ) • J. Russell • S. Živanovic´ • J.T. Mottram School of Engineering, University of Warwick, Coventry, CV4 7AL, UK e-mail: x.wei.3@warwick.ac.uk © The Society for Experimental Mechanics, Inc. 2017 J. Caicedo, S. Pakzad (eds.), Dynamics of Civil Structures, Volume 2, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-54777-0_5 37
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